I use a big programmable Casio fx-9750g, which is fantastic - solves simultaneous equations, does matrix algebra, all sorts of things (cost me about £45, but well worth it). My spare is the current Casio fx-83, which is nearly as good for everyday use, cost about 8 quid, just won't handle much in the way of degree level maths (being a lazy sod, I hate solving quadratics and simultaneous equations longhand).
On the other hand the CAA insists that for exams you're only allowed a little 8-digit display, non programmable, probably-came-free-with-petrol calculator. Up to date as ever! So, if you are practicing for exams, you may as buy a simple one with big buttons and pictures of the red arrows on the top at an airshow.
G